If you stay with sprint these are the things you need to look for in a sprint phone
1) Make sure the phone is spark compatible, this means the phone is using the most frequencies that Sprint is capable to use. Spark is a marketing term made by sprint to advertise those phones have the greatest frequency range. More frequency range means faster speeds and bettter signal for your phone.
Spark phones are able to use the following frequencies for lte/4g 800MHz, 1900MHz, 2500MHz. Another way to say those frequencies is called the lte bands so look for phones that can use band 25 (1900 mhz), band 26 (800 mhz), and band 41 (2500 mhz).
2) Make sure the phone is using the snapdragon 800 or 801 cpu/soc/chipset. This means you are going to get the fastest computer speed on the market right now. The snapdragon 800/801 also had much better battery life measurements than the older snapdragon cpus.
3) Form Factor, make sure you find the phone comfortable and how it fits in your hand. Sure a bigger screen or larger battery has it advantages but the phone needs to be comfortable to use.
4) Make sure the screen is at least 720p, while 1080p screens have slight improvements in sharpness most people can't tell the difference.
5) Make sure the battery life is going to be good enough for you.
6) Price. If you can get a lot better deal on the phone by getting x phone vs y phone and it meats the previous 5 criteria then go for it.
7) Everything else. Things such as camera, speakers, etc all phones are slightly different and some are better than others in marginal ways, every phone you get will better than the htc evo, htc evo 3d, or htc evo lte in every way. We are talking about a huge upgrade.
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Or just get an Iphone 5s, that said the new iphone will be out in about 4 months and will have a bigger screen
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Good phones
Nexus 5
Moto X
HTC One M8
Samsung Galaxy S5
LG G2 (the new LG G3 will be coming out soon)
There are probably more that I am forgetting, each of the above phones have their own advantages and disadvantages. Now I am not sure if all the phones listed above are spark compatible, if I recall they are all new enough that they should be but I am not a 100% sure and I haven't done my own research on the frequency to double check. Now for example the samsung galaxy s4 was not originally spark compatible for it is too old but sprint released an updated version where they had qualcomm the person who put the cellphone chip inside it with an updated cellphone radio chip which could access more frequencies thus if you buy a samsung galaxy s4 used or refurbished you may or may not have the spark compatible s4.
One nice thing about the Nexus 5 besides price, speed, features, is that google purposefully made it a carrier unlocked device as well as both cdma and gsm compatible so one phone in the us works on sprint, t-mobile, and at&t. Thus if you get a nexus 5 you can use it on sprint and then if you want to try another network or provider you can just swap the sim card and the phone works thus you can for example try out the $30 dollar walmart t-mobile plan and see if the coverage in your area is good.
Most other phones are purposefully dumb down with either the limited cell phone radio chips and/or carrier hardware locks so you do not try to switch carriers and keep the phone or even try out the competition.